Laser separation of the epitaxial layers of the semiconductor crystals from the growth substrates with their transfer on to the ungrowth substrates is widely used now in manufacturing of the light diodes (U.S. Pat. No. 7,241,667, U.S. Pat. No. 7,202,141) and laser diodes (U.S. Pat. No. 6,365,429) according to the flip-chip technology.
In the first time, laser separation of the gallium nitride layers from the transparent growth sapphire substrates was proposed in the work Kelly et al Physica Status Solidi (a) vol. 159, pp. R3, R4, (1997). In this work the ultraviolet excimer laser with wave length λ=355 nm satisfying the ratio 2πhc/Eg1<λ<2πhc/Eg2 was used, for which the quantum energy is within the forbidden gap of the substrate Eg1 made of sapphire, but exceeds the width of the forbidden gap of the epitaxial film Eg2, consisting of gallium nitride.
Later, the method of laser separation based on the difference between the widths of the forbidden gaps of the growth substrate and epitaxial film was improved. In particular, to improve quality of the separated epitaxial film and to suppress its cracking, it was proposed to use additional sacrifial layer with the width of the forbidden gap less than the widths of the forbidden gaps of the growth substrate and epitaxial film, as well to use scanning of the heteroepitaxial interface between the growth substrate and epitaxial film (U.S. Pat. No. 6,071,795, U.S. Pat. No. 6,365,429).
The general scheme of the laser separation methods based on the difference between the widths of the forbidden gaps of the growth substrate and epitaxial film is shown in FIG. 1.
When exposing to ultraviolet from the side of the substrate from heteroepitaxial semiconductor gallium nitride film 102 grown on the substrate 101 of sapphire having the width of the forbidden gap more than the light quantum energy, ultraviolet laser radiation passes through sapphire and is absorbed in the thin layer of gallium nitride nearby the heteroepitaxial interface 105 gallium nitride—sapphire. On exposure to ultraviolet laser radiation, gallium nitride in the area 104 defined by crossing of the ultraviolet laser radiation 103 with heteroepitaxial interface 105 is heated up to the temperature T1, exceeding the decomposition temperature T0˜900° C., and decomposes into gaseous nitrogen and liquid gallium, and as a result epitaxial film of gallium nitride separates from sapphire.
All before proposed methods of laser separation of epitaxial films from the growth substrates are based on the difference between the widths of the forbidden gaps of the epitaxial film Eg2 and substrate Eg1. These methods can be successfully used for separating the epitaxial films obtained using heteroepitaxy method, i.e., technology of growing the epitaxial film onto the growth substrate made of the material which differs from the epitaxial film material.
However, to obtain high quality epitaxial films without integrated mechanical stresses, it is often happened to be necessary to use a homoepitaxy method, which provides growing of the epitaxial film on the growth substrate from the same material as the epitaxial film. In this case growth substrate and epitaxial film have an equal width of the forbidden gap, and the usual laser separation method disclosed above becomes unapplied.
The object of the present invention is an expansion of the method application field, namely providing the possibility of separating the epitaxial films from the substrates made of the same crystal material as the epitaxial film.